Newer-generation magnetic resonance imaging (MRI) systems may generate and transmit a plurality of individual radio-frequency (RF) pulse trains in parallel over different independent radio-frequency transmit channels. Individual RF signals are applied to the individual transmit channels (e.g., the individual rods of a whole-body antenna).
Parallel transmission techniques, however, may increase peak pulse power, giving rise to concerns regarding excessive exposure to RF energy. The RF energy from an MRI scan may cause heating of the tissue of a body. One measure of RF absorption is the specific absorption rate (SAR), which specifies the deposited power per unit mass (watts/kg) due to the RF pulse. Inhomogeneity of an RF field (e.g., generated by the whole-body antenna) leads to one or more local exposures (e.g., hot spots) where a majority of the absorbed energy is applied (e.g., local SAR). Maximum values for SAR are specified by safety regulations and are to be met both globally (e.g., power absorbed by the whole body, such as the head of a patient) and locally (e.g., power absorbed per 10 grams of tissue). For example, a standardized limit of 4 watts/kg applies to the global SAR of a patient, and a standardized limit of 10 watts/kg applies to the local SAR for, for example, the head of the patient, according to an International Electrotechnical Commission (IEC) standard.
The value of SAR depends on the geometry of the part of the body that is exposed to the RF energy and the location and geometry of the RF source (e.g., the rods of the whole-body antenna). The SAR is based on the frequency-dependent conductivity of the tissue, the electric field, the tissue density, and scales with the square of the main magnetic field and with the square of the flip angle. Global SAR may be calculated by averaging the RF power absorbed per unit mass over the exposed tissue (e.g., the whole body). The global SAR may be monitored in real-time such that the global SAR does not exceed the standardized limit.
Online or real-time local SAR monitors for monitoring local SAR have not been used. Instead, a consistency check of played-out HF pulses is performed using set-point values.